E-cadherin is a transmembrane molecule that forms a protein complex with cytoplasmic catenins in the zonula adherens of epithelial cells, where it has an established function in cell-cell adhesion. β-catenin binds via its armadillo-repeats directly to the C-terminal tail of E-cadherin (Stappert and Kemler, 1994). The vinculin homolog α-catenin establishes a link to the actin cytoskeleton by binding to the N-terminal part of the E-cadherin-bound β-catenin on the one hand (Jou et al., 1995), and to F-actin or an actin-bound α-actinin dimer on the other hand (Knudsen et al., 1995; Rimm et al., 1995). Proper formation of this E-cadherin/catenin-complex has been shown to be crucial for normal early embryonic development as well as for the maintenance of differentiation, polarization and integrity of adult epithelial tissue structures (Behrens et al., 1989; McNeill et al., 1990). p120ctn is another Armadillo catenin that binds to the membrane-proximal cytoplasmic part of E-cadherin (Daniel and Reynolds, 1995), which is involved in the establishment of strong E-cadherin-mediated cell-cell adhesion (Thoreson et al., 2000). As the presence of a functional E-cadherin/catenin-complex is a prerequisite for normal development and maintenance of epithelial structures in the mammalian body, acquisition of molecular abnormalities in one of the elements of this complex are related to the development and progression of epithelial cell-derived tumors, i.e., carcinomas.
Suppression of the E-cadherin/catenin-complex leads to invasion and metastasis. E-cadherin has been shown to be a potent invasion suppressor (Frixen et al., 1991; Vleminckx et al., 1991) as well as a genuine tumor suppressor (Berx et al., 1995; Berx et al., 1996). Loss of E-cadherin expression is reported for at least fifteen types of carcinomas (Potter et al., 1999) and is correlated with the loss of intercellular adhesion, increased cellular motility, changes in the organization of the actin filaments and a scattered growth pattern of the carcinoma cells (Handschuh et al., 1999). PCT International Patent Publication No. WO9411401 claims, amongst others, the use of E-cadherin to treat malignancies and to detect metastatic potential. PCT International Patent Publication No. WO9920168 describes the analysis of germline mutations for detecting predisposition to cancer. Besides mutational inactivation of the E-cadherin gene, which is so far restricted to infiltrative lobular breast and diffuse gastric carcinomas (Becker et al., 1994; Berx et al., 1996; Berx et al., 1998), transcriptional downregulation is the major cause of loss of E-cadherin expression in human carcinomas. PCT International Patent Publication No. WO0102860 describes the use of Snail, a transcription factor that acts as a repressor of the expression of E-cadherin, in tumor control and as diagnostic marker. As catenins are indispensable for E-cadherin functionality, loss of α-catenin or β-catenin also induces invasion of carcinoma cells (Vermeulen et al., 1999).
The key question is whether the observed role of the E-cadherin/catenin-complex in tumor growth and invasion is the direct result of its function in cell-cell adhesion or whether a more complex signaling pathway may be involved. Indeed, β-catenin and p120ctn can, when they are not bound to E-cadherin, translocate to the nucleus where they bind via their armadillo-repeats to the transcription factors LEF-1 and Kaiso, respectively (Behrens et al., 1996; Daniel and Reynolds, 1999; Huber et al., 1996). In particular, the formation of the β-catenin/LEF-1 heterodimer and the subsequent effect on transcriptional regulation are the main events of the transmission of the canonical Wnt signaling cascade to the nucleus (Miller et al., 1999). In this facet of β-catenin function, β-catenin is part of another cytoplasmic multiprotein complex, consisting of APC (Adenomatous Polyposis Coli protein), axin or conductin, and GSK3β. Without Wnt signal, β-catenin in this complex is phosphorylated by GSK3β on specific Ser-residues and in this way targeted for ubiquitin-triggered degradation. Upon binding of secreted Wnt molecules to their transmembrane Frizzled receptors, Disheveled protein will inhibit the kinase GSK3β. This results in the stabilization of cytoplasmic β-catenin that now can translocate to the nucleus and bind LEF-1. E-cadherin and LEF-1 form mutually exclusive complexes with β-catenin. E-cadherin has the potent ability to recruit β-catenin to the cell membrane and to prevent in this way its nuclear localization and transactivation activity (Orsulic et al., 1999; Sadot et al., 1998). On the contrary, E-cadherin may regulate the activity of β-catenin through mechanisms other than this canonical membrane sequestration/nuclear localization (Gottardi et al., 2001).
Recently, a novel phosphorylation-independent pathway for β-catenin degradation was described, affecting the activity of β-catenin-dependent transcription (Liu et al., 2001; Matsuzawa and Reed, 2001). In the latter pathway, β-catenin is part of yet another multiprotein complex involving Siah-1 binding to APC. Siah-1, the mammalian product of a p53 inducible growth arrest gene, is the homolog of the Drosophila sina (seven in absentia) gene (Hu et al., 1997). In order to target other proteins for ubiquitin-proteasome-mediated degradation, Siah-1 binds target proteins via its carboxy-terminal domain while association with ubiquitin-conjugating enzymes occurs via an amino-terminal RING domain (Hu and Fearon, 1999). Abnormal stabilization of β-catenin was shown to be involved in tumorigenesis (Gumbiner, 1997; Morin et al., 1997; Peifer, 1997; Rubinfeld et al., 1997). For colon cancer in particular, oncogenic forms of β-catenin were found in which GSK3β-targeted Ser residues are lost by mutation, thus preventing degradation of cytoplasmic and nuclear β-catenin and leading to activated Tcf/LEF target genes. Also truncation mutations of the APC gene were reported to yield the same stabilizing effect on β-catenin as these truncated APC molecules lost their β-catenin binding sites.
Also the binding of E-cadherin to p120ctn has been shown to be mutually exclusive with the interaction of p120ctn with Kaiso (Daniel and Reynolds, 1999). Like for β-catenin, E-cadherin has the potent ability to recruit p120ctn to the cell membrane and to prevent in this way its nuclear localization and potential transactivation activity (van Hengel et al., 1999). Moreover, p120ctn overexpression disrupts actin stress fibers, which correlates with reduced Rho activity (Anastasiadis et al., 2000; Noren et al., 2000). Also E-cadherin binding and the ability of p120ctn to affect Rho are mutually exclusive events (Anastasiadis and Reynolds, 2001). Assuming that an equilibrium exists between the cadherin-bound pool and the cytosolic pool of p120ctn, E-cadherin expression could regulate Rho-activity and hence actin reorganization and cell motility via p120ctn.